In general, infrared radiation (IR) sensors are used in a variety of applications to detect infrared radiation and provide an electrical output that is a measure of the incident infrared radiation. One type of infrared sensor is a bolometer. A bolometer includes a sensor structure configured to absorb infrared radiation and a transducer element in thermal contact with the sensor structure. The transducer element has an electrical resistance that varies with temperature. Infrared radiation incident upon the bolometer will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained.
The sensitivity of a bolometer depends at least partially on how well the absorber and transducer elements of the sensor is thermally isolated from surrounding structures. The sensitivity generally increases with better thermal isolation. The sensor structure is usually suspended above the surface of the substrate to minimize heat loss through thermal contact with the substrate. To prevent heat losses through convection, vacuum encapsulation of the sensor structure is often required. In previously known fabrication processes, vacuum encapsulation of the sensor structure was accomplished on chip level using, for example, metal can packages. Chip-level vacuum encapsulation, however, is a costly packaging process, which is difficult to scale up for very large volume fabrication.
Recent developments have enabled sensor structures to be vacuum encapsulated on a wafer level with wafer bonding. In wafer bonding, a cap wafer is bonded onto the sensor wafer over the sensor structure. However, wafer bonding requires the use of a bond frame in the cap wafer which can significantly increase the size of the device. In addition, for wafer bonding, the thickness of the encapsulating structure has strict lower limits due to mechanical stiffness requirements for handling of the cap wafer. As a result, the encapsulating structure is thicker than would otherwise be required which results in increased absorption losses of the infrared radiation traveling through the cap.